Photo-conductive elements have been known for some time--see, for example, "Electronic Devices and Circuits" by Jacob Millman and Christos C. Halkias, McGraw Hill Book Company, 1966. Other articles or publications dealing with photo-conductive elements are, for example, Photocell design theory and application, from "Photoconductive Cell Applications Design Handbook" Clairex Electronics, 1978, Vactec Inc., Bulletin PCD-6, 1970, "Electrooptics Information Sheet, CdS CDSe Photoconductive cells" Centronic Inc. 1101 Bristol road, Mountainside N.J. 07092, and from RCA Transistor and Thyristor Diode Manual, RCA, 1969. Information on lumped circuit elements (LE) can be obtained, for example, from "Advances in Microwaves" Edited by Leo Young, Academic Press, Vol. 8, New York, N.Y, while information on conventionally constructed directive antenna arrays can be obtained, for example, from the book "Antenna Theory and Design" by Warren L. Stutzman, and Gary A. Thiele, John Wiley & Sons.
When light falls on the photo-conductive element, generally a semiconductor material, its resistance decreases, so that its electrical conductivity increases. The fact that this phenomenon relies on a bulk effect means that no PN junction is required for its operation. Many semiconductor materials suitable for use in the photo-conductive element permit a change in resistivity extending over about 5 orders of magnitude.
If a photo-conductive material of this type were to be placed in the channel of a coplanar microwave capacitor, alterations in the real component of the capacitor's impedance could be obtained. However, a difficulty arises due to the fact that at microwave frequencies, for example of the order of 10 GHz, the capacitive reactance X.sub.c of the coplanar capacitor is much lower than the lowest resistance R offered by the photo-conductive element. Thus, although the real component R of the impedance of the coplanar microwave capacitor can be made to vary over the aforecited relatively large range, this variation is masked by the low value of capacitive reactance X.sub.c in parallel with the resistance R. Due to the aforecited masking it has not been possible hitherto to use a photo-conductive element at microwave frequencies for controlling applications at such frequencies.